/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.lucene.search.grouping;

import java.io.IOException;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.search.Collector;
import org.apache.lucene.search.LeafCollector;
import org.apache.lucene.search.Scorable;

Concrete implementations of this class define what to collect for individual groups during the second-pass of a grouping search. Each group is assigned a Collector returned by newCollector(), and LeafCollector.collect(int) is called for each document that is in a group 
Type parameters:
<T> – the type of the value used for grouping
<C> – the type of Collector used to reduce each group
See Also:
SecondPassGroupingCollector/**
 * Concrete implementations of this class define what to collect for individual
 * groups during the second-pass of a grouping search.
 *
 * Each group is assigned a Collector returned by {@link #newCollector()}, and
 * {@link LeafCollector#collect(int)} is called for each document that is in
 * a group
 *
 * @see SecondPassGroupingCollector
 *
 * @param <T> the type of the value used for grouping
 * @param <C> the type of {@link Collector} used to reduce each group
 */
public abstract class GroupReducer<T, C extends Collector> {

  private final Map<T, GroupCollector<C>> groups = new HashMap<>();

  
Define which groups should be reduced. Called by SecondPassGroupingCollector /**
   * Define which groups should be reduced.
   *
   * Called by {@link SecondPassGroupingCollector}
   */
  public void setGroups(Collection<SearchGroup<T>> groups) {
    for (SearchGroup<T> group : groups) {
      this.groups.put(group.groupValue, new GroupCollector<>(newCollector()));
    }
  }

  
Whether or not this reducer requires collected documents to be scored
/**
   * Whether or not this reducer requires collected documents to be scored
   */
  public abstract boolean needsScores();

  
Creates a new Collector for each group
/**
   * Creates a new Collector for each group
   */
  protected abstract C newCollector();

  
Get the Collector for a given group
/**
   * Get the Collector for a given group
   */
  public final C getCollector(T value) {
    return groups.get(value).collector;
  }

  
Collect a given document into a given group
Throws:
IOException – on error/**
   * Collect a given document into a given group
   * @throws IOException on error
   */
  public final void collect(T value, int doc) throws IOException {
    GroupCollector<C> collector = groups.get(value);
    collector.leafCollector.collect(doc);
  }

  
Set the Scorer on all group collectors
/**
   * Set the Scorer on all group collectors
   */
  public final void setScorer(Scorable scorer) throws IOException {
    for (GroupCollector<C> collector : groups.values()) {
      collector.leafCollector.setScorer(scorer);
    }
  }

  
Called when the parent SecondPassGroupingCollector moves to a new segment /**
   * Called when the parent {@link SecondPassGroupingCollector} moves to a new segment
   */
  public final void setNextReader(LeafReaderContext ctx) throws IOException {
    for (GroupCollector<C> collector : groups.values()) {
      collector.leafCollector = collector.collector.getLeafCollector(ctx);
    }
  }

  private static final class GroupCollector<C extends Collector> {

    final C collector;
    LeafCollector leafCollector;

    private GroupCollector(C collector) {
      this.collector = collector;
    }
  }

}